Furnace for Continuous Gas Curing, Especially of Rubber Products

ABSTRACT

A continuous gas baking furnace includes elements for heating a baking area through which the product which is to be treated passes, formed by at least one module including a heat exchanger with a gas burner and elements for propelling and directing heated air towards a baking area, wherein the gas combustion circuit of the heat exchanger is separate from the circuit of hot air flowing through the baking area, the elements for directing the hot air include paths for directing hot air on both sides of the baking area and a path for evacuating air via the middle of the baking area. The furnace includes members for ensuring pyrolysis of gaseous effluents, the members treating a fraction of the gas circulating in a given section of the furnace, and at least one catalytic bed which is arranged on the hot air circuit.

The present invention concerns curing furnaces and relates, inparticular, to furnaces for curing rubber products.

Known electric curing furnaces have reasonable construction costs buthigh operating costs.

Also known are gas curing furnaces which consume a very large amount ofpower and have the following technical drawbacks:

poor insulation resulting in needless consumption,

evacuation of the combustion gases released by the gas burner throughthe heating stream, entailing risks of pollution of the products to becured,

simultaneous evacuation by mechanical extraction of the curing vapourreleased by the product and the combustion gases released by the heatingmeans resulting in excess power consumption; it is therefore necessaryto evacuate a large amount of hot air to extract the burnt gases even ifthe product does not release curing steam,

non-uniform air stream over the length of the furnace resulting incuring distributed non-uniformly over the product; this can cause heatshocks,

mechanical extraction which is dependent simultaneously or separately onthe curing vapour and the combustion gases, thus rendering it difficultto adjust and optimise the curing.

In order to overcome these drawbacks there has been proposed a furnaceconfiguration which is suitable, in particular but not exclusively, forthe curing of rubber for the vulcanisation thereof, which configuration,despite having lower operating costs than known furnaces, has improvedoutput and facilities for adjusting the curing parameters.

This furnace for the curing of rubber products comprises means for thegas heating of a curing area through which the product to be treatedpasses. The furnace is formed by juxtaposed modules integrated in aproduction line and each comprising a suitably insulated housing, a heatexchanger with a gas burner and means for propelling and directing airheated by the heat exchanger toward an area for curing the product, thegas combustion circuit of the heat exchanger being separate from thecircuit of hot air passing through the curing area. The means fordirecting hot air toward the curing area comprise paths for directinghot air on both sides of the curing area and a path for evacuating airthrough the middle of the curing area.

This furnace is described in document EP-A-1 216 129.

During the vulcanisation of rubber there are produced polluting andtoxic gaseous effluents such as nitrosamines, carbon monoxide andnitrogen oxides. Obviously, it is not desirable to release theseeffluents into the atmosphere either during or after vulcanisation ofthe products to be treated. These effluents have to be treated inadvance in an installation dedicated to this operation. This treatmentinstallation substantially increases the overall size and the cost ofthe furnace and its attachments.

The object of the invention is to propose a modification to theabove-described curing furnace which, while preserving its optimumcharacteristics from the point of view of the curing of the products tobe treated, also allows it to treat the polluting emissions underexcellent technical and economic conditions.

The invention accordingly relates to a furnace for continuous gascuring, especially of rubber products, comprising means for heating acuring area through which the product to be treated passes, formed by amodule or a plurality of juxtaposed modules, integrated within aproduction line and each comprising a suitably insulated housing, a heatexchanger with a gas burner and means for propelling and directing theheated air through the heat exchanger toward an area for curing theproduct, the gas combustion circuit of the heat exchanger being separatefrom the circuit of hot air flowing through the curing area, the meansfor directing the hot air also being distributed and set apart by gridsalong the curing area comprising paths for directing hot air on bothsides of the curing area and a path for evacuating the air via themiddle of the curing area, characterised in that it also comprises:

means for pyrolysing gaseous effluents present in the air circuit andresulting from the curing of the product, said means treating a fractionof the gases travelling in a given portion of the furnace;

and at least one catalytic bed interposed on the hot air circuit.

Said means for pyrolysing the gaseous effluents resulting from theproduct may comprise means for adjusting said fraction of the gases thatis treated by pyrolysis.

The heat exchanger of each furnace module may be an exchanger with ahairpin tube which is internally heated using a corresponding gasburner, the means for propelling and directing hot air toward the curingarea of each module may comprise at least one ventilation turbinearranged in proximity to the bottom of the housing of the module facingpaths for directing hot air which are delimited by metal sheets fordeflecting hot air to direct it either side of the curing area, andgrids for distributing hot air along the curing area, and said means forpyrolysing said gaseous effluents may comprise a casing which is open atits two ends surrounding said hairpin tube and is positioned in thevicinity of said turbine, separated from said tube by a gap andterminated facing said turbine.

Said casing may be provided with means for varying its distance from theturbine.

Said casing may be provided with means for varying its length.

Said catalytic bed may be based on one or more precious metals depositedon an inorganic support.

As will be clear, the invention consists in interposing on the pathitself of the gases travelling in the furnace of the type describedhereinbefore:

on the one hand, means for pyrolysing the gaseous effluents which arecapable of being treated in this way, said pyrolysis using as a heatsource the heat exchanger of the furnace itself;

on the other hand, at least one catalytic bed capable of catalysing thegaseous effluents which can be treated in this way and were produced bythe vulcanisation or pyrolysis referred to hereinbefore.

In a favoured variation of the invention, the gaseous effluents aretreated by pyrolysis as a result of the fact that the heat exchanger isa hairpin exchanger and that, in proximity to the ventilator causing thegases to travel in the furnace, a branch of the hairpin is surrounded bya casing which is open at its two ends and assists the passage of aportion of the gas travelling in immediate proximity to the exchanger.The pyrolysis is thus carried out using extremely simple means which mayeasily be adapted to an existing furnace.

The invention will be better understood on reading the followingdescription given with reference to the attached figures. In thedrawings:

FIG. 1 is a schematic diagram of an example of a gas furnace to whichthe invention may be adapted;

FIG. 2 is a partially exploded perspective view of the furnace from FIG.1;

FIG. 3 is a perspective view of a portion of a hairpin heat exchangerfor the furnace from FIGS. 1 and 2, equipped with means according to theinvention pyrolysing the travelling gases; and

FIG. 4 is a cross-section along the plane 4-4 from FIG. 2.

The following description relates to an embodiment of the inventionbased on a development of the furnace described in the above-citeddocument EP-A-1 216 129. For further details concerning thecharacteristics and the operation of this furnace, reference mayusefully be made to said document. Nevertheless, it will be understoodthat this specific embodiment of the invention does not entail anylimitation.

The furnace shown schematically in FIG. 1 comprises a plurality offurnace modules 1, 2, 3, 4, the number of which is dependent on thespeed of travel of the product to be cured, the speed of travel beingdetermined by the speed of the upstream production line and theresidence time required for the product to cure.

The modules 1, 2, 3, 4 are joined together by means for assembling thehousings thereof, appropriate heat seals 64, 65, 66 being interposed.

Associated with each module 1 to 4 is a gas burner 5, 6, 7, 8 which issupplied with gas via a supply conduit 9, 10, 11, 12 and internallyheats a stainless steel heat exchanger 13, 14, 15, 16 formed by ahairpin tube. Each burner produces, from a mixture of air and gas, aflame which extends inside the tube and then conveys the combustionproducts without any contact with the exterior of the tube. Thecombustion products are drawn in by an extraction ventilator at theoutput of the tube.

In order to simplify the construction of the furnace, the heatexchangers are symmetrically inverted in the region of the junctionsbetween the modules, thus allowing assembly of the gas supply conduits 9and 10 and the conduits 17, 18, 19 and 20 for extraction of burnt gases,the extraction conduits being connected to evacuation chimneys 22, 23rising above the roof.

As may be seen more clearly from FIG. 2, each module comprises a housing24, 25, 26 in which the corresponding exchanger 13, 14, 15 is mounted.

Each housing comprises, like the housing 25 shown in greater detail inFIG. 2, a lateral access door 27 which is intended for furnacemaintenance and extends, in the region of the exchangers 13, 14, 15, 16,over the entire length of the module.

Each module also comprises an upper door 28 for accessing a curingtunnel in which there is arranged a roller conveyor 29 carried by tworails 30 and arranged above the region of the heat exchanger 15.

Opening the doors 28 allows complete access over the entire length ofthe furnace, facilitating maintenance of the curing area (cleaning orreplacing a mat or rollers of the conveyor) and guiding of the productto be cured during start-up of a production operation.

Instead of a roller conveyor, it is possible to use a perforated ormeshed mat (not shown).

In the wall of the housing 25 that is remote from the maintenance accessdoor 27 there is arranged a ventilation turbine 32 which is driven inrotation by an electric motor 33 equipped with a cooling turbine 34intended to dissipate the heat transmitted by the transmission shaftconnecting the ventilation turbine and the electric motor and having oneend inside the furnace and one end outside the furnace.

This rules out the risk of an excessive rise in the temperature of theelectric motor.

The ventilation turbine 32, which is advantageously made of stainlesssteel, is arranged in the lower portion of the wall of the housing 26that is remote from the door 27.

The number and the position of the ventilation turbines 32 may vary asrequired and in accordance with the overall design of the furnace. Theremay be one or more turbines per module 1, 2, 3, 4.

Owing to the central extraction of steam by the device, to whichreference will be made hereinafter, and to these turbines 32, there isdrawn in outside air which penetrates the furnace via the orifice forfeeding the product to be cured into the first module 1 and via theorifice in the last module 4 via which the cured product leaves thefurnace; this drawn-in air also progresses in this way toward thecentral area of the furnace.

Between the turbine 32 and the area of the roller conveyor there arearranged metal deflection sheets 35, 36, 37, made for example ofstainless steel, which define paths 38, 39 of heating air indicated bycorresponding arrows, said paths 38, 39 ending either side of the rollerconveyor 29 in order uniformly to heat the product conveyed by theconveyor and to provide a return path 40 toward the bottom of thehousing.

The heating air paths 38, 39 open onto lateral grids 41, 42 arrangedeither side of the rollers 29, whereas the return path 40 is defined bya series of air inlets 44 located below the rollers 29.

The air inlets 44 pass through the path 39 and end at the base of thehousing between the metal deflection sheets 36 and 37. Owing to theadvancement of the hot air brought about by the turbines 32, there isthus obtained a helical configuration of the path of the air within thefurnace. As it advances, this air becomes filled with a broad range ofgases resulting from the curing of the product. These are gases to betreated within the scope of the invention.

The furnace may be equipped on one of its lateral walls with an orifice54 opening into a conduit 55 for evacuating the curing gases travellingtherein. This orifice 54 and the corresponding evacuation conduit 55 arehighly preferably arranged in the central position relative to thefurnace as a whole. If the furnace comprises an odd number of modules,the orifice 54 and the conduit 55 are formed in the middle of thecentral module. If the furnace comprises an even number of modules (asshown in the figures), the orifice 54 and the conduit 55 are (as shownin FIG. 1) formed in the region of the heat seal 65 connecting the twomost central modules 2, 3. Instead of this, there may also be providedtwo orifices 54 each formed on one of the modules 2, 3, in proximity tothe end thereof closest to the seal separating the modules, and theevacuation conduits issuing from these orifices 54 may be joined to forma single evacuation conduit. If slight decentering of the evacuation ofthe curing gases is deemed to be tolerable, there may also be providedmerely a single orifice 54 formed on the wall of one of the two modules2, 3 in proximity to the end thereof closest to the middle of thefurnace. These last two configurations rule out problems concerning thetightness of the heat seal 65 which might result from the formation ofan orifice 54 therein and the connection of a conduit 55 thereto.

This central arrangement of the evacuation of the gases provides a highdegree of symmetry and a high degree of uniformity of the flow of curingarea within the furnace, and also satisfactory drawing ensuring aneffective exchange of heat, on the one hand, between the air intendedfor curing and the exchangers and, on the other hand, between the airintended for curing and the product to be cured. The helicalconfiguration of the path of curing air that is provided by the metaldeflection sheets 35, 36, 37 and the air inlets 44 and the drawingassisted by the turbine or turbines 32, causing the curing air to followthis helical path, help to obtain this result.

It may therefore be seen that the circuit for directing and evacuatingthe heating air from the product displaced on the roller conveyor 29 isindependent of the gas combustion circuit in the burners.

The walls and the doors of the housing 25, like those of the otherhousings, are in the form of an insulating material formed from ceramicfibres with crosswise layers arranged between an inner coating 31 a inthe form of a stainless steel sheet and an outer coating 31 b made ofpainted steel.

The doors, such as the lateral door 27 and the upper door 28 for accessto the curing tunnel, are equipped, as are the frames thereof, withrespective seals 48, 49 and 50, 51 in the form of refractory braids.

As shown in FIG. 3, depending on the nature, the amount and the toxicityof the curing steam released by the product, a burning device 53 mayoptionally be installed on an evacuation conduit 55. A gas burner 57 ispositioned in the evacuation conduit 55 and projects its flame in thedirection of flow. A bell 59 then caps the discharge conduit above theburner 57, thus forming a heating chamber. The burnt gases are thencollected around this bell 59 by a chamber 60, then drawn in through theconduit 61 by an extraction ventilator 58. A viewing port 62 formed inthe chamber 60 allows inspection of the clogging of said chamber withsoot or the like. The gas burner 57 has to be capable of bringing thesteam to a temperature of approximately 1,200° C.

Depending on the nature and the characteristics of the product to becured, a device for regulating the humidity of the hot air required forcuring may be installed to prevent any drying-out of the product to becured or to modify the surface appearance of the product after curing(brightness, orange peel effect, etc.). As soon as it is detected thatthe moisture content of the curing air is too low, an adjustable amountof water will be poured into a tank 63 positioned on the bottom of thefurnace. Once this water has evaporated, the amount of water vapourcontained in the curing air will return to an adjustable humidity level.

The furnace operates as follows.

When a product to be heated is placed at the input of the furnace, theoperator opens the upper doors 28 for access to the curing tunnel andthe front end of a product to be continuously cured is positioned in thecuring tunnel. The operator then closes the doors 28.

Once the furnace has been adjusted in advance as a function of thecuring parameters of the product to be treated, it is started up and theproduct is driven in the roller conveyor 29 at the desired speed whichis compatible with the residence time of the product in the furnace.

The gas burners 5, 6, 7, 8 heat the air entering the heat exchangers 13,14, 15, 16. The flames of the burners spread in the hairpin tubes of theexchanger and the burnt gases are evacuated via the chimneys 22, 23(FIG. 1) without any contact with the air for heating the product.

For each module 1 to 4, a turbine 32 draws in the hot air preceding theheat exchanger 15 via its centre and dispels the air over its entireperiphery. A plurality of turbines 32 may be provided instead of justone. In this case, the deflector 35 separates the flow of air propelledin two air streams 38 and 39 having identical speeds.

The air stream 38 guided by the walls 31 a and 35 then passes throughthe lateral grids 41 in order to spread its heat uniformly andmoderately in the curing area over the entire length of the right-handside of the furnace.

Similarly, the air stream 39 guided by the walls 35 and 36 then passesthrough the lateral grids 42 in order to spread its heat uniformly andmoderately in the curing area over the entire length of the left-handside of the furnace.

A large number of air inlets 44 located below the conveyor 29 thencombine the air streams 38 and 39 into a single return air stream 40toward the heat exchanger 15 in order to be reheated.

If the furnace is equipped with a fume burning device, a portion of thegas which is caused to move by the turbines is evacuated from thehousing of the furnace via the conduit 54 (FIG. 3), the fumes are burntin the vertical conduit 56 by the gas burner 57 and evacuated throughthe baffle formed by the bell 59 and the chamber 60 toward the chimney61 at a rate which is dependent on the rotational speed of theventilator 58.

According to the invention, the above-described furnace also comprisesthe following elements (not shown in FIGS. 1 and 2).

In the areas adjacent to a turbine 32, the branch 15 a of the hairpin 15of the heat exchanger that is closest to the turbine 32 is surrounded bya casing 67 which terminates facing the turbine 32.

The purpose of this casing 67 (which in the illustrated example extendseither side of the turbine 32 and terminates facing said turbine 32) isto allow a fraction of the gases travelling in the furnace to be drawninto the gap 68, which may have a width of for example from about 5 to50 mm, separating the casing 67 and the branch 15 a of the hairpin 15.The arrows 69, 70, 71, 72, 73 in FIGS. 3 and 4 indicate this travel.

In this way, said fraction of the travelling gases filled with curingvapour passes closest to the exchanger 15 and is subjected, in aconfined space, to the intense heat radiation released by the exchanger,especially as the branch 15 a which is closest to the turbine 32 is (inthe construction of the furnace provided by way of example) the branchclosest to the burner 7 and therefore the hottest branch.

As the turbine 32 creates a reduction in pressure in the area whichfaces it and in which the casing 67 terminates, there is thereforegenerated a natural draught between the branch 15 a of the exchanger 15and the casing 67, allowing a significant fraction of the gas flowtravelling in the relevant portion of the furnace to pass through thegap 68. The amount of this fraction may be adjusted if appropriate, byvarying the rotational speed of the turbine 32 and/or the distancesbetween the turbine 32 and the ends of the two portions of the casing 67if displacement of these portions is controllable and/or of the lengthof the casing 67 if this is variable during use of the furnace (forexample, by making the casing telescopic and by associating therewithappropriate control means).

The remainder of the gas flow travels in the furnace and passes aboveand below the casing 67. As the casing is made of a heat-conductingmaterial, it is brought to a temperature similar to that of theexchanger 15 and is also involved in the heating of the gases which lickthe casing in the directions indicated by the arrows 74, 75 in FIG. 4.

The heat conditions prevailing in the gap 68 promote pyrolysis of thevapour released by curing. This pyrolysis requires temperatures of fromabout 350 to 800° C., more generally from 400 to 700° C. and morespecifically from 500 to 600° C., in the case of the vapour resultingfrom the curing of the rubber.

As the actual curing of the rubber has to be carried out at atemperature of about merely 300° C., it will be understood that it wouldnot be desirable simultaneously to treat by pyrolysis all of the gasflow travelling in the furnace. Such treatment would rapidly bring allof the gas flow to a temperature which exceeds this order of magnitudeand is therefore unsuitable for curing the products. This is why asubstantial portion of the gas flow is left free to travel outside thecasing 67 in order to be maintained at the normal temperature for curingthe products. In practice, the installation is configured andoperatively regulated so that approximately, for example, 20% of theoverall gas flow stirred in the corresponding area of the furnace passesthrough the gap 68 and is subjected to pyrolysis before being redilutedin the main flow.

As the furnace operates in a closed circuit, as the gaseous effluentspass repeatedly before one of the exchangers 15, both outside and insidethe casing 67, all of the gases which result from the curing of theproducts and are capable of being pyrolysed end up being treated,although this does not result in excessive heating of the gases involvedin curing.

More specifically, it is the nitrosamines derived from the vulcanisationthat have to be destroyed by the pyrolysis.

This pyrolysis, but also the curing of the products, generates pollutingeffluents which have to be destroyed before the gases travelling in thefurnace are dispelled into the atmosphere. In order to carry out thisdestruction within the actual interior of the furnace, as theseeffluents are produced, the invention provides for the gases travellingin the furnace also to pass through at least one catalytic bed 76, 76′.This catalytic bed must, in particular, be suitable for treating the COand nitrogen oxides produced by the vulcanisation of the rubber productsand by the above-described pyrolysis.

This catalytic bed 76, 76′ may be implanted at any desired location ofthe path of the gaseous effluents travelling in the furnace, for examplewith reference to the type of furnace illustrated in the figures:

above the turbine 32 and below the deflector 35 so as to treat therising gas flow before it separates into two streams 38, 39 orientedeither side of the roller conveyor 29; this is the case for the bed 76from FIG. 4;

or on the return path 40 toward the bottom of the housing of the gaseswhich have passed through the roller conveyor 29; this is the case forthe bed 76′ from FIG. 4;

or immediately upstream of the turbine 32;

or at any other locations in the furnace through which there passes aflow of travelling gases;

or at a plurality of these locations simultaneously.

Typically, the catalysts of the catalytic bed or beds 76, 76′ are basedon one or more precious metals deposited on various inorganic supports,typically caesium, rhodium, platinum, palladium. Platinum and palladium,and preferably palladium, are the preferred examples thereof. Treated ornon-treated aluminas, various kieselguhrs, ceramics or any availableinorganic chemical forms may be used as the supports.

These supported catalysts may be in all the various possible geometricalforms, for example in the form of perforated or non-perforated hollowtubes of all sizes, all diameters and all lengths with perforations ofall possible dimensions and all possible shapes.

The catalysts are typically in the form of powders and preferably in theform of spherical balls of all diameters and all specificcharacteristics in terms of porosity, specific surface area, density,wear, and all of the other characteristics of the supported catalyststhat enable them to transform by catalysis the vapour emitted duringcuring the products which the furnace has to treat and during pyrolysisof this vapour. The content of the above-cited precious metals maytypically be between 0.01 and 5% by weight, generally between 0.1 and 2%by weight and preferably between 0.3 and 1% by weight.

The furnace which has been described and illustrated hereinbefore ismerely one possible embodiment of the invention. It will be understood,in particular, that the gaseous effluents could be pyrolysed by meansother than the casing 67 surrounding the branch 15 a of the exchanger15, especially if use is made of a type of heat exchanger other than ahairpin exchanger.

It is preferable for each module 1, 2, 3, 4 of the furnace to beequipped with pyrolysis means and catalysis means even if, strictlyspeaking, it might be sufficient to provide merely a single module owingto the fact that the gases travel within the furnace.

It will be understood that the presence 54 of the evacuation conduit 55and the members associated therewith is not compulsory.

Obviously, the invention would also apply to a furnace composed merelyof a single module.

1. Furnace for continuous gas curing, especially of rubber products,comprising means for heating a curing area (29) through which theproduct to be treated passes, characterised in that it is formed by amodule or a plurality of juxtaposed modules (1, 2, 3, 4), integratedwithin a production line and each comprising a suitably insulatedhousing (24, 25, 26), a heat exchanger (13, 14, 15, 16) with a gasburner (5, 6, 7, 8) and means (32, 33, 35, 36, 41, 42) for propellingand directing the heated air through the heat exchanger toward an area(29) for curing of the product, the gas combustion circuit of the heatexchanger being separate from the circuit of hot air flowing through thecuring area, the means for directing the hot air also being distributedand set apart by grids (41, 42) along the curing area comprising paths(38, 39) for directing hot air on both sides of the curing area (29) anda path (40) for evacuating the air via the middle of the curing area,characterised in that it also comprises: means for pyrolysing gaseouseffluents present in the air circuit and resulting from the curing ofthe product, said means treating a fraction of the gases travelling in agiven portion of the furnace; and at least one catalytic bed (76, 76′)interposed on the hot air circuit.
 2. Gas furnace according to claim 1,characterised in that said means for pyrolysing the gaseous effluentsresulting from the product comprise means for adjusting said fraction ofthe gases that is treated by pyrolysis.
 3. Curing furnace according toclaim 1, characterised in that the heat exchanger (13, 14, 15, 16) ofeach furnace module is an exchanger with a hairpin tube which isinternally heated using a corresponding gas burner (4, 5, 6, 7), in thatthe means for propelling and directing hot air toward the curing area ofeach module comprise at least one ventilation turbine (32) arranged inproximity to the bottom of the housing of the module facing paths fordirecting hot air which are delimited by metal sheets (35, 36) fordeflecting hot air to direct it either side of the curing area, andgrids (42, 42) for distributing hot air along the curing area, and inthat said means for pyrolysing said gaseous effluents comprise a casing(67) which is open at its two ends surrounding said hairpin tube (15 a)and is positioned in the vicinity of said turbine, separated from saidtube by a gap (68) and terminated facing said turbine (32).
 4. Furnaceaccording to claim 3, characterised in that said casing (67) is providedwith means for varying its distance from the turbine (32).
 5. Furnaceaccording to claim 3, characterised in that said casing (67) is providedwith means for varying its length.
 6. Furnace according to claim 1,characterised in that said catalytic bed (76, 76′) is based on one ormore precious metals deposited on an inorganic support.
 7. Curingfurnace according to claim 2, characterised in that the heat exchanger(13, 14, 15, 16) of each furnace module is an exchanger with a hairpintube which is internally heated using a corresponding gas burner (4, 5,6, 7), in that the means for propelling and directing hot air toward thecuring area of each module comprise at least one ventilation turbine(32) arranged in proximity to the bottom of the housing of the modulefacing paths for directing hot air which are delimited by metal sheets(35, 36) for deflecting hot air to direct it either side of the curingarea, and grids (42, 42) for distributing hot air along the curing area,and in that said means for pyrolysing said gaseous effluents comprise acasing (67) which is open at its two ends surrounding said hairpin tube(15 a) and is positioned in the vicinity of said turbine, separated fromsaid tube by a gap (68) and terminated facing said turbine (32). 8.Furnace according to claim 7, characterised in that said casing (67) isprovided with means for varying its distance from the turbine (32). 9.Furnace according to claim 4, characterised in that said casing (67) isprovided with means for varying its length.
 10. Furnace according toclaim 2, characterised in that said catalytic bed (76, 76′) is based onone or more precious metals deposited on an inorganic support. 11.Furnace according to claim 3, characterised in that said catalytic bed(76, 76′) is based on one or more precious metals deposited on aninorganic support.
 12. Furnace according to claim 4, characterised inthat said catalytic bed (76, 76′) is based on one or more preciousmetals deposited on an inorganic support.
 13. Furnace according to claim5, characterised in that said catalytic bed (76, 76′) is based on one ormore precious metals deposited on an inorganic support.